In the last decade physicists have observed that when polarized electromagnetic radiation of a selected wavelength is coupled to the surface of a thin metal layer forming an interface with a dielectric medium a portion of the electromagnetic radiation is reflected while an evanescent portion of the electromagnetic radiation (referred to as a surface plasma wave or surface plasmon or by the acronym SP) is propagated along the interface of the metal and dielectric medium.
In some instances an electrooptic dielectric medium has been employed. With a properly selected angle of incidence it is possible by electrically varying the refractive index of the electrooptic medium to vary the proportion of incident electromagnetic radiation that is reflected or internally propagated as surface plasmons. When the metal layer is positioned between the electrooptic medium and a dielectric layer, the thicknesses of the dielectric and metal layers are selected as a function of the wavelength of the electromagnetic radiation, and the indices of refraction of the dielectric layer and electrooptic medium match at least approximately, it is possible to increase the internal propagation sensitivity of the device to differences in electrical biasing of the electrooptic medium efficiency by coupling the evanescent portion of the incident electromagnetic radiation at the two interfaces of the metal layer into an antisymmetric mode, referred to as a long range surface plasmon (LRSP). When efficient long range surface plasmon coupling is achieved, possible within only a narrow range of electrical biasing, a very low proportion of incident electromagnetic radiation is reflected. A long range surface plasmon device can be modulated similarly to a surface plasmon device, but with higher variations in reflected electromagnetic radiation being realizable for a given variance in applied voltage.
Despite a consensus on the physics of operation, actual surface plasmon devices and, particularly, long range surface plasmon devices, which place even more stringent requirements device construction, have been disclosed in forms that demonstrate theoretical feasibility, but fall well short of being practically attractive to construct and use.
Sincerbox et al. U.S. Pat. No. 4,249,796, issued Feb. 10, 1981, is illustrative. Sincerbox's best mode of constructing a surface plasmon modulator is to optically couple a LaSF.sub.5 prism (refractive index, n=1.88) to a sapphire plate (n=1.77) through an index matching liquid. A silver layer having a thickness of 300 to 500 .ANG. serves as the reflective metal layer. An aqueous solution of 0.3 M KBr and 0.0113 M heptylviologen bromide completes a conductive bridge to a counter electrode. Notice that the sapphire plate serves as the support for the silver layer and that two separate liquid couplings are required to complete the device. It should be further noted that Sincerbox contains no suggestion of a long range surface plasmon modulator.
Sarid, "Long-Range Plasmon Waves on Very Thin Metal Films", Phys. Rev. Lett., Vol. 47, No. 26, pp. 1927-1930 (1981), describes long range surface plasmon propagation in a theoretical manner, but offers no suggestion as to how such a device could be constructed.
McNeill et al. U.S. Pat. No. 4,451,123, issued May 29, 1984, discloses a device similar to that of Sincerbox et al., but differing in the variable refractive index medium employed. For this purpose McNeill et al. employs a doped semiconductor capable of forming a rectifying junction with the metal film. The device operates in a bistable switching mode. In the absence of an applied electrical bias across the the semiconductor the device is "on", meaning that incident collimated electromagnetic radiation striking the base of the prism is reflected. When an electrical bias is applied, the refractive index of the semiconductor adjacent its interface with the metal film is altered, resulting in surface plasmon generation at the interface, which reduces reflected radiation and turns the device "off". The surface plasmon device is either "on" or "off", has no image forming capability, and does not lend itself to conversion to a long range surface plasmon device.
Yang et al., "Long-Range Surface Modes of Metal-Clad Four-Layer Waveguides", Applied Optics, Vol. 25, No. 21, pp. 3903-3908 (1986), is cumulative with Sarid in its theoretical discussion of long range surface plasmons, but goes somewhat further in reporting an actual device construction. A silver film of from 100 to 250.ANG. in thickness was evaporated on a "Ag.sup.+ exchanged glass waveguide" not otherwise identified. A prism made of ZF7 glass (n.sub.p =1.7997) was coupled to the silver layer through an index matching liquid composed of naphthalene bromide and coal oil. Modulation was achieved by squeezing the device to change the thickness of the liquid layer.
Plumereau et al., "Electrooptic Light Modulator Using Long-Range Surface Plasmons", SPIE, Vol. 800, Novel Optoelectronic Devices, pp. 79-83 (1987), is cumulative with Sarid and Yang et al. in its theoretical discussion of long range surface plasmons, but provides in FIG. 1 a sketch of a constructed device consisting of a TiO.sub.2 prism (1), an Ag layer (2), a CuCl layer (3), an Ag layer (4) and a CuCl layer (5). Modulation is achieved by applying a voltage between (2) and (4). Few clues as to actual device construction are provided beyond the indication that the electrooptic CuCl layer was monocrystalline with a {111] crystallographic orientation. It was suggested that zinc oxide could be used in place of CuCl as an electrooptic material. A very narrow angular range of &lt;10.sup.-2 degrees produced the resonance required for long range surface plasmon generation.
Persegol et al., "A Novel Type of Light Modulator", SPIE Vol. 864, Advanced Optoelectronic Technology, pp. 42-44 (1987), discloses in FIG. 1 a silicon support having a 2000 .ANG. silica layer which is in turn coated with a 6930 .ANG. zinc oxide layer, coated with a 95 .ANG. gold layer. The device is completed by mounting a prism spaced from the gold layer by an air gap. Modulation is achieved by placing an electrical bias between the gold layer and the silicon substrate.
Schildkraut, "Long Range Surface Plasmon Electrooptic Modulator", Applied Physics, vol. 27, No. 21, Nov. 1, 1988, pp. 4587-4590, discloses in FIG. 1 a long range surface plasmon generator. Schildkraut reports no actual device construction, but basis calculations on the assumption that electrooptic film is modeled as a noncentrosymmetric organic film having a .chi..sup.(2) zzz=2.times.10.sup.-7 esu.
Yeatman et al., "Surface Plasmon Spatial Light Modulators", SPIE, Vol. 1151, Optical Information Processing Systems and Architecture, pp. 522-532 (1989), suggests the use of a surface plasmon device as a spatial light modulator (SLM). In a broad theoretical sense this is achieved merely by segmenting the counter electrode so that each segment can be separately biased for imaging purposes. In an experimental construction, shown in FIG. 5, a silver layer is coated on the base of high index prism and glass slide and a liquid crystal composition is confined between the silver layer and a counter electrode with thin magnesium fluoride alignment layers being interposed. The counter electrode is divided into segments. A Mylar.TM. spacer of from 6 to 10 .mu.m in thickness is glued between the counter electrode and silver layer to confine the liquid crystal composition. Yeatman et al. suggests alternatively employing a semiconductor depletion region or a Langmuir-Blodgett (LB) film as a replacement for the liquid crystal electrooptic medium, contemplated constructions of each being shown in FIGS. 8 and 9, respectively. Yeatman et al. does not address the construction of long range surface plasmon spatial light modulators.